1. Field of the Invention
The present invention is generally in the field of communications devices. More specifically, the invention is in the field of power amplifiers for communications devices.
2. Related Art
Wireless communication devices, such as cellular handsets, generally utilize two classes of power amplifiers (“PA”): linear PAs, where output power is controlled by input power; and saturated PAs, where input power is constant and output power is controlled by a control voltage, such as an analog power control voltage (“voltage for analog power control” or “VAPC”). Linear PAs are used in, for example, cellular handsets using code-division multiple access (“CDMA”) wireless communication standard. Cellular handsets using global system for mobile communications (“GSM”) wireless communication standard that use the enhanced data through GSM evolution (“EDGE”) modulation format also require a linear PA.
To ensure proper linear operation, a linear PA requires an appropriate DC bias, which can be provided by using a conventional current mirror circuit including a current source, a reference transistor, and a voltage follower transistor. In a PA comprising a radio frequency (“RF”) output transistor, a base-emitter voltage can be provided at the base of the RF output transistor by coupling the base of the RF output transistor to the base of the reference transistor and utilizing the current source to inject a known current into the collector of the reference transistor. The voltage follower transistor, which is coupled to the bases of the reference and RF output transistors, provides the necessary base current to the reference transistor and the RF output transistor. As a result, a constant DC bias is generated by the conventional current mirror circuit to appropriately bias the linear PA. In the linear PA discussed above, the output power of the RF output transistor is proportional to the input power of an RF input signal coupled to the base of the RF output transistor.
Saturated PAs are used in, for example, cellular handsets using a GSM wireless communication standard that utilizes Gaussian minimum shift keying (“GMSK”) modulation. In a saturated PA, the phase of RF input signal is varied to transmit information while the power of the RF input signal is held constant. In a saturated PA comprising an RF output transistor, DC bias can be provided by a bias circuit comprising a control voltage, such as VAPC, coupled to the base of the RF output transistor via a resistor. In the saturated PA, the output power of the RF output transistor is a monotonic function of VAPC applied to the base of the RF output transistor.
As discussed above, the saturated PA and the linear PA each require a different bias circuit. However, it is desirable for a PA to be able to operate in both a saturated mode and a linear mode to support multi-mode wireless applications.
Thus, there is a need in the art for a low-cost, easy to implement bias circuit that can effectively support both linear and saturated operating modes of a power amplifier.